Absolute measurement scale system

ABSTRACT

A scale system for absolute measurement includes at least one scale (1&#39;) which extends along a measuring length, and at least one measuring slide which is movable in relation to the scale. The system includes a fine sensor which functions to measure the absolute values within each of the intervals lying along the scale (1&#39;), and further includes a rough sensor which functions to measure the absolute values with regard to the interval in which the fine sensor is located at that time. The scale system further includes at least one rough scale (2&#39;, 3&#39;) whose scale electrodes (2a, b, 3a, b) define a predetermined angle to the direction in which a measuring slide is moved along the scale, wherein the measuring electrodes of the measuring slide also define a preferably corresponding angle (α) with the direction of slide movement, whereby a determined displacement of the slide in the slide movement direction will be corresponded by a determined relative movement between the measuring electrodes of the measuring slide and the scale electrodes of the rough scale in a direction perpendicular to the extension of the scale electrodes, this direction defining a predetermined angle with the aforesaid direction of slide movement.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a scale system for absolutemeasurements, including at least one scale which extends along ameasuring length and a measuring slide which is movable in relation tothe scale, a fine sensor which functions to measure the absolute valueswithin each of the mutually sequential intervals or graduations alongthe scale, a rough sensor which functions to measure the absolute valueswith respect to that interval or graduation within which the fine sensoris located at that particular time, and wherein each scale includesscale electrodes which are disposed in a predetermined pattern and whichare intended to coact with measuring electrodes mounted at respectivemeasuring heads, wherein a supply voltage is applied to the measuringelectrodes to achieve capacitive measuring, and wherein said scalesystem further comprises a signal includes unit.

2. Description of the Related Art

Systems of this kind are known to the art. However, such known systemsare encumbered with serious drawbacks. In known systems which "switch"between two scales that have mutually different graduations, theelectrodes must be spaced apart extremely accurately if a comparativelylarge total measuring length is to be achieved. In other known systemswhich employ a photoelectric rough scale based on the so-called graycode, it is necessary to use highly complicated measuring heads whichneed to surround the scale and which include a light-transmitting uniton one side of the scale and a photoreceptive array on the other sidethereof. In order to obtain a reasonable scale length, some knownsystems also require a measuring slide which has a long axial extensionin the direction of movement of the slide.

SUMMARY OF THE INVENTION

The present invention relates to an absolute, capacitive measuringsystem which, among other things, enables long measuring lengths to beobtained without requiring the electrodes to be spaced apart with highprecision and which will also enable relatively simple and shortmeasuring slides to be used. The invention also provides several otherimportant improvements to the known prior art.

The present invention thus relates to an absolute measurement scalesystem which includes at least one scale that extends along a measuringlength, at least one measuring slide which is movable in relation to thescale, a fine sensor for measuring the absolute values within each ofthe graduations along the scale, a coarse sensor for measuring theabsolute values with regard to the interval in which the fine sensor islocated at that particular moment, and a signal processing unit, whereineach scale includes scale electrodes which are disposed in apredetermined pattern and which coact with measuring electrodes mountedadjacent a measuring slide, and wherein a supply voltage is applied tothe measuring electrodes for capacitive measurement.

The system is mainly characterized in that there is provided at leastone rough scale whose scale electrodes define a predetermined angle (α)with the direction of movement of a measuring slide along said scale; inthat the measuring electrodes of the measuring slide also preferablydefine a corresponding angle (α) with said direction of slide movement,whereby a given linear displacement (L) in said movement direction iscorresponded by a given relative linear displacement (S) between themeasuring slide electrodes and the rough scale electrodes in a direction(α') perpendicular to the extension of the scale electrodes, saiddirection defining a predetermined angle with said direction of lineardisplacement.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference toexemplifying embodiments thereof and also with reference to theaccompanying drawings, in which

FIG. 1 illustrates schematically part of a fine scale and a measuringslide of a known scale system;

FIG. 2 illustrates diagrammatically the principles of a rough scale anda measuring slide according to the present invention;

FIG. 3 illustrates schematically a first embodiment of a rough scale,measuring slide and signal transmission in accordance with the presentinvention;

FIG. 4 illustrates schematically a second embodiment of a rough scale,etc., in accordance with the invention;

FIG. 5 illustrates schematically two different types of rough scaleshaving mutually different period lengths;

FIG. 6 illustrates two scales according to FIG. 5 in a compact,superposed configuration;

FIG. 7 illustrates schematically two differentially arranged roughscales, where the scales are mutually mirrored in a longitudinallyextending plane;

FIG. 8 illustrates two scales according to FIG. 7 where supply is incounterphase between the scales;

FIG. 9 illustrates counterphase supply in signal form;

FIG. 10 illustrates counterphase supply according to FIGS. 8 and 9 inmeasuring electrode form with scales according to FIG. 8;

FIG. 11a illustrates schematically two fine scales where the scales aremutually phase-shifted and FIG. 11b two fine scales where thetransmission parts of the scale electrodes are mutually joined; and

FIG. 12 illustrates schematically part of an inventive scale systemaccording to a third embodiment, this embodiment being preferred in somecases.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The known scale system illustrated in FIG. 1 is described in detail inSwedish Patent Specification No. 7714010-1 and includes, among otherthings, a graduated scale 1 which is provided with a first system ofscale electrodes 2, 3 and a measuring slide 4 which is displaceablealong the scale and which is provided with a second electrode system5-8. The electrode spacing, or electrode division, is chosen so that

    p=P/n

where

p=the electrode spacing of the slide;

P=the electrode spacing of the scale; and

n=the number of electrodes in each phase group on the slide.

The direction of slide movement is shown by the arrow 9. Each electrodeof an electrode group is supplied with an alternating voltage inaccordance with a cyclic pattern. For the purpose of amplifying theuseful signal, there is normally included a plurality ofparallel-coupled phase groups on the slide 4, and in order for the scalesystem to operate efficiently, the slide must include at least one wholephase group of measuring electrodes 5-8.

Since the length of a phase group is directly connected with the lengthof the scale period, the greatest permitted length of the slide willalso determine the greatest permitted length of the scale period. Forpractical reasons, among others, the slide cannot be permitted to beexcessively long. A maximum length is preferably about 50-100 mm. Themaximum working range of the slide is thus about 100 mm, whichunfortunately is much too small for the majority of applications.

The principle, inventive scale system illustrated in FIG. 2 alsoincludes a measuring slide 10 provided with measuring electrodes 11-14.The direction in which the slide is moved is shown by the arrow 15. TheFIG. 2 system also includes a rough scale 18 which is provided withscale electrodes 16, 17 which define a predetermined acute angle α withthe aforesaid direction of slide movement 15, where the measuringelectrodes 11-14 of the measuring slide also define a predeterminedangle with said movement direction 15, this angle preferablycorresponding to the angle α. Thus, a given movement distance L of theslide in said movement direction 15 will correspond with a givenrelative movement distances between the measuring electrodes 11-14 ofthe measuring slide and the scale electrodes 16, 17 of the rough scalein a direction α' perpendicular to the extension of the scale electrodes16, 17, this direction defining a predetermined angle α+90° with theaforesaid movement direction, as will be evident from FIG. 2.

FIGS. 3 and 4 illustrate exemplifying embodiments of the invention inmore detail. The reference numeral 19 identifies a scale which isprovided with a plurality of, preferably equidistant, scale electrodes20, 21. A measuring slide 22 mounted on the scale 19 is movable alongthe scale and carries a plurality of measuring electrodes 23 (11-14),transmitter electrodes 23 and, in the illustrated case, FIG. 3, receiverelectrodes 24, which are arranged in a determined pattern. As beforementioned, each of the measuring electrodes is supplied with analternating voltage in accordance with a cyclic pattern. The scaleelectrodes include transmission parts 26 and measuring parts 25 and thereceiver electrodes 24 are intended to capture through the agency ofsaid transmission parts 26 signals received by said measuring parts 25and to transmit these signals to a signal processing unit 27 connectedto the receiver electrodes.

Thus, the rough scale measuring electrodes 20, 21 include a measuringpart 25 which defines said angle α with said movement direction 15, anda transmission part 26. In the case of the FIG. 3 embodiment, thistransmission part 26, and similarly the measuring part 25, are separateand electrically isolated from the transmission parts and measuringparts of remaining scale electrodes, and transmission part 26 areintended to be sensed by means of at least one of the receiverelectrodes carried by the measuring slide, from which receiver electrodesignals are transmitted to the signal processing unit 27. In the case ofthe FIG. 4 embodiment, the transmission part 26 is connected to andforms part of a long transmission part 28, which is common to severalscale electrodes and which is connected directly to the signalprocessing unit 27.

The embodiment illustrated in FIG. 5 includes two types of rough scale19a and 19b which have mutually different period lengths L_(a), L_(b)wherein the measuring process "switches" between the two scales so as toachieve an unequivocal determination of the absolute positions along thescales over a determined distance or period length M, in accordance withthe equation ##EQU1##

For instance, at L_(a) =131.072 mm and L_(b) =135.168 mm, phase M willequal 4194 mm which is sufficient for the majority of sizes ofcoordinate measuring machines or machining tools for instance.Naturally, the choice of L_(a) and/or L_(b) may be different from theaforesaid values so that M will be commensurately larger or smaller.

The scale configuration illustrated in FIG. 5 has a pronounced extension6 transversely to the longitudinal axis of the scales. According to oneembodiment, FIG. 6, which is preferred in some cases, the two roughscales 19a and 19b of a pair of rough scales of mutually different typesand of different period lengths are arranged within one another, ininterchanging relationship with their respective transmission parts onopposite sides of the scale configuration which includes the two scales,therewith to obtain a compact scale system. In the FIG. 6 configuration,one and the same array of measuring electrodes 23 on the slide can beused to deliver signals to both scales.

As will be seen from FIG. 7, in order to further restrict sensitivityfor movements perpendicular to the direction of slide movement, forinstance as a result of obliqueness of the measuring parts of the roughscales, there is provided for each rough scale of determined periodlength, a first scale 29, a further, corresponding second rough scale29', which is a mirror image of the first rough scale 29 in a planewhich is parallel with the aforesaid movement direction 18. A measuringslide 30 includes measuring electrodes 23' for the second rough scale29', these electrodes being a mirror image of the measuring electrodes23 of the first rough scale 29, similar to the mirror-imaged scales 29,29', and wherein the signals deriving from the two rough scales arecoprocessed to eliminate measuring errors deriving, for instance, fromobliqueness of the rough scales. Thus, when the slide 30 is moved in thedirection 15, the measuring electrode groups 23, 23' on the slide willbe located simultaneously in the same position in relation to themeasuring parts 25, 25' of the scale electrodes. On the other hand, inthe event of relative movement between the slide and the scales in adirection Y transversely to the longitudinal axis of the scales, theupper measuring electrode group 23, which in the FIG. 7 example is in aposition with the S-phase immediately above the measuring part 25, willbe moved to a position where the R-phase comes progressively more intoeffect. In its initial position, the mirrored electrode group 23' isalso located in a position with the S-phase immediately above themeasuring part 25', and is displaced to a position in which the T-phaseis brought further into effect. Since the R-phase and T-phase aremutually phase-shifted through 180°, any error contributions will canceleach other out. The arrangement illustrated in FIG. 7 thus renders thescale system insensitive to any movement in the Y-direction and to anylack of parallelity between scale 29, 29' and the movement direction 15of the measuring system.

When mutually joined transmission parts 28, FIGS. 4 and 8, of the scalesare used, there is a risk that these transmission parts will formtogether with the scale electrodes "antennas" which capture externalelectric interference, for instance interference signals from electricmotors, lighting systems and the like. This problem is overcome with apreferred embodiment of the invention illustrated in FIGS. 8 and 9,according to which the power supply for the measuring electrodes 23' forthe one 29' of the scales in a pair 29, 29' of mirrored scales isarranged for counterphase signals R', S', T', U' compared with thesignals R, S, T, U for the other scale 29, so that the power suppliesfor mutually corresponding measuring electrodes will be phase-shiftedthrough 180° relative to one another, wherein an operational amplifier27' is provided for processing the signals of respective scales 29, 29'.The operational amplifier 27' and the transmission parts 28, 28' arearranged so that one common transmission part 28 is connected to thepositive input (+) of the amplifier, whereas the other transmission part28', delivering counterphase signals, is connected to the negative input(-) of the amplifier, so that both signals, for instance S and S' asshown in FIG. 8, will contribute equally to forming the output signalS_(ut) from the operational amplifier. As opposed to the measuringsignal, any interference signals will enter on both of the transmissionparts 28, 28' with the same phase and direction (sense) and willtherefore be suppressed in the amplifier 27'. The amplifier used willpreferably be a high so-called common mode rejection amplifier.

As will be seen from FIG. 9, the 180° phase-shifted signals in a 4-phasesupply system will result only in a 180° shift of the phase order in themirrored array of measuring electrodes on the slide, wherein, as shownin FIG. 9, phase R' is identical with phase T, phase S' is identicalwith phase U, and so on. The relationship between counterphase and phaseis thus:

    R'=T

    S'=U

    T'=R

    U'=S

The final phase supply picture shown in FIG. 10 is obtained when R', S',T' and U' are replaced with their equivalents according to the abovetable.

The preferred configuration of the fine scale illustrated in FIGS. 11aand 11b includes two scales a, b, where the scale electrodes of eachscale have a common transmission part 31, which preferably extends overthe full length of the scale, i.e. the scale electrodes are continuousover the full length of the scale. It is preferred in this case thateach scale is connected directly to the signal processing unit 27. Thescales a, b are preferably mutually phase-shifted through 180°, wherebytwo differential signals are obtained so as to enable externalelectrical interferences and disturbances to be suppressed with the aidof an operational amplifier 27' having so-called common mode rejection.

FIG. 12 illustrates another embodiment of rough scales which includes afirst type of rough scale 2a, 2b of relatively short period length, forinstance a period length of 131.072 mm, and a second type of rough scale3a, 3b having a relatively long period length, in many cases preferablythe full measuring length, for instance 2000 mm. Each rough scale 2', 3'is provided with differential scales and with differential, powersupplies as described with reference to FIGS. 7, 8 and 9. In the case ofthis system, absolute measurement is effected by means of the fine scalescale 1' within its scale period, for instance 2.048 mm, and such thatthe position of the fine period concerned within a scale period of themost finely graduated rough scale 2' can be established with said roughscale 2', and so that the position of the shorter rough scale periodconcerned within the rough scale having the longest scale period can beestablished by means of the rough scale 3' having the longest scaleperiod.

The manner in which the inventive scale system works will be evidentessentially from the aforegoing. Thus, a fundamental feature of theinventive system is that the scale electrodes of the rough scale orscales are angled in relation to the movement direction 15, wherebyrelative movement a distance L in that direction, FIG. 2, will alsoresult in relative movement between the measuring electrodes of themeasuring slide and the electrodes of the scale in the movementsensitive direction α' of the system, wherein α can be chosen so as toenable a desired transmission ratio between the movements in directions15 and α' to be disengaged in principle from the movement in thedirection 15 and adapted to current requirements concerning resolution,acceptable slide length, etc.

It will also be evident that the present invention provides importantadvantages over the known technique. Less precision is required withrespect to scale electrode spacing. The measuring slide can be given asimple construction. The system can be given a large absolute measuringrange without needing to increase the length of the measuring slide tounreasonable limits. Other advantages are also afforded.

Although the invention has been described in the aforegoing withreference to exemplifying embodiments thereof, it will be understoodthat other embodiments and minor changes are conceivable within theconcept of the invention.

For instance, according to the preferred embodiments, the scale periodof the fine scale is 2,048 mm. Furthermore, the resolution of the signalprocessing unit is 1/4096 of the scale electrode spacing or division inthe α'-direction, this spacing being 4,096 mm in the case of thepreferred embodiments. The resolution is thus 1 μm in said direction.With regard to the most finely graduated rough scale, α is preferablychosen so that L, i.e. the length travelled in the movement direction15, FIG. 2, will be a binary multiple, 64, of the period length of thefine scale, 2.048 mm. In this case, α is 1.7908°. One unit in theresolution of the signal processing unit will then give a resolution inthe slide movement direction 15, according to ##EQU2## There is obtainedan absolute measuring length of 64×2,048=131,072 mm with a measurementresolution of 32 μm over this length.

It will understood that the invention is not restricted to theaforedescribed and illustrated embodiments thereof and thatmodifications and changes can be made within the scope of the followingclaims.

We claim:
 1. A scale system comprising: at least one scale extendingalong a measuring length, at least one measuring slide which is movablein relation to the scale along a direction of movement, a fine sensorfor measuring absolute position values within distance intervals lyingalong the scale, a coarse sensor for measuring the absolute positionvalues with regard to an interval within which the fine sensor islocated at a particular moment, and a signal processing unit forreceiving position signals, wherein each scale includes scale electrodeswhich are disposed in a predetermined pattern and which coact withmeasuring electrodes mounted for movement with the measuring slide, andincluding a power supply for providing a supply voltage to measuringelectrodes carried by the measuring slide for capacitive measurement ofposition, wherein each scale further includes at least one rough scalehaving a plurality of scale electrodes that extend at a predeterminedacute angle relative to the direction of movement of the measuringslide, wherein the measuring electrodes extend at the same predeterminedacute angle relative to the direction of slide movement as the scaleelectrodes, whereby a given linear displacement of the slide in saidslide movement direction results in a corresponding linear displacementof the measuring slide electrodes relative to the scale electrodes ofthe rough scale in a direction perpendicular to the acute angle betweenthe scale electrodes and the direction of movement of the slide.
 2. Asystem according to claim 1, wherein the scale electrodes of the roughscale include a measuring part which extends at said acute anglerelative to said slide movement direction, and a transmission part thatis separate from transmission parts of other scale electrodes and issensed by the measuring electrodes carried by the measuring slide,wherein the measuring electrodes provide measurement signals to a signalprocessing unit.
 3. A system according to claim 1, including two opposedrough scales which each have mutually different period lengths, whereinalternations of measurements obtained from each of the two rough scalesdetermine absolute positions over a determined distance, wherein thepredetermined distance is defined according to the relationship ##EQU3##wherein M is the predetermined distance, L_(a) is the period length ofone period for one of the opposed rough scales and L_(b) is the periodlength of one period for the other of the opposed rough scales.
 4. Asystem according to claim 3, wherein the two rough scales are disposedin interengaging relationship so as to obtain a scale system of smallsize.
 5. A system according to claim 1, wherein each rough scaleincludes a first rough scale and a second rough scale, wherein thesecond rough scale is a mirror image of the first rough scale in a planewhich extends parallel with said movement direction and which isperpendicular to a plane containing the first rough scale; wherein thesecond rough scale includes measuring electrodes that are a mirror imageof the measuring electrodes of the first rough scale, so that whenmeasurement signals from the two scales are combined measuring errorswhich derive from measuring slide movement in a direction transverse tosaid measuring slide movement direction are eliminated.
 6. A systemaccording to claim 5, wherein a power source is coupled with themeasuring electrodes of of the mirror-imaged scales and the power sourceprovides to the measuring electrodes of each rough scale power that isphase-shifted through 180° relative to the phase of the power providedto the other rough scale; and wherein the system includes an operationalamplifier for processing measurement signals derived from respectivescale electrodes to provide common mode rejection of unwanted signalsfor eliminating interference.
 7. A system according to claim 1, whereinthe rough scale includes a first rough scale having a relatively shortperiod length and a second rough scale having a relatively long periodlength, wherein the period length of the second rough scale has the samelength as the measuring length of the system.
 8. A scale systemaccording to claim 1, including a fine scale defined by a plurality ofspaced fine scale electrodes that extend from and that are spaced alonga transmission part, and a signal processing unit coupled with thetransmission part for receiving and processing signals provided by thetransmission part during a measuring operation.
 9. A scale systemaccording to claim 8, wherein the fine scale includes two fine scaleswhich are mutually phase-shifted by 18020 ; and wherein the systemincludes an operational amplifier coupled with each of the two finescales for processing measurement signals provided by the two finescales during a measuring operation to suppress external electricalinterferences that are common to the measurement signals from therespective two fine scales.
 10. A system according to claim 1, whereinthe scale electrodes of the rough scale include a measuring part whichextends at said acute angle relative to said slide movement direction,and a transmission part that is connected with correspondingtransmission parts of other rough scale electrodes to define a commontransmission part, wherein the common transmission part is coupled witha signal processing unit for processing signals from the commontransmission part.